Rotating Control Device with Mechanical Seal

ABSTRACT

A rotating control device for use in a drilling system, the rotating control device comprising a non-rotating tubular RCD housing enclosing an elongate passage, a mandrel which extends along the elongate passage, the mandrel having an axis and being configured in use to rotate about said axis, and a seal assembly which is configured to provide a substantially fluid tight seal between the RCD housing and the mandrel, wherein the seal assembly comprises a first seal ring which is fixed around an exterior face of the mandrel for rotation with the mandrel, and a second seal ring which is secured to the RCD housing, the first and second seal rings each having a sealing face, and a biasing assembly which acts on one of the seal rings to push the sealing faces together by means of a biasing force which is generally parallel to the axis of the mandrel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States non-provisional application of and claims priority to United Kingdom Patent Application No. 1900372.2, filed Jan. 11, 2019, which claims priority to U.S. Provisional Patent Application Ser. No. 62/777,867, filed on Dec. 11, 2018.

FIELD OF INVENTION

This invention relates in general to fluid drilling equipment and in particular to a rotating control device (RCD) to be used for pressurized drilling operations. More specifically, embodiments of the present disclosure relate to a RCD having a mechanical seal assembly that increases bearing performance and life by ensuring a reliable seal from wellbore pressure.

BACKGROUND OF INVENTION

In drilling a well, a drilling tool or “drill bit” is rotated under an axial load within a bore hole. The drill bit is attached to the bottom of a string of threadably connected tubulars or “drill pipe” located in the bore hole. The drill pipe is rotated at the surface of the well by an applied torque which is transferred by the drill pipe to the drill bit. As the bore hole is drilled, the hole bored by the drill bit is substantially greater than the diameter of the drill pipe. To assist in lubricating the drill bit, drilling fluid or gas is pumped down the drill pipe. The fluid jets out of the drill bit, flowing back up to the surface through the annulus between the wall of the bore hole and the drill pipe.

Conventional oilfield drilling typically uses hydrostatic pressure generated by the density of the drilling fluid or mud in the wellbore in addition to the pressure developed by pumping of the fluid to the borehole. However, some fluid reservoirs are considered economically undrillable with these conventional techniques. New and improved techniques, such as underbalanced drilling and managed pressure drilling, have been used successfully throughout the world. Managed pressure drilling is an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore. The annular pressure profile is controlled in such a way that the well is either balanced at all times, or nearly balanced with low change in pressure. Underbalanced drilling is drilling with the hydrostatic head of the drilling fluid intentionally designed to be lower than the pressure of the formations being drilled. The hydrostatic head of the fluid may naturally be less than the formation pressure, or it can be induced to be less.

Rotating control devices provide a means of sealing off the annulus around the drill pipe as the drill pipe rotates and translates axially down the well while including a side outlet through which the return drilling fluid is diverted. Such rotating control devices may also be referred to as rotating blow out preventers, rotating diverters or drilling heads. These units generally comprise a stationary housing or bowl including a side outlet for connection to a fluid return line and an inlet flange for locating the unit on a blow out preventer or other drilling stack at the surface of the well bore. Within the bowl, opposite the inlet flange, is arranged a rotatable assembly such as anti-friction bearings which allow the drill pipe, located through the head, to rotate and slide. The assembly includes a seal onto the drill pipe which is typically made from rubber, polyurethane or other suitable elastomer.

For offshore application on jack-up drilling rigs or floating drilling rigs the rotating control device may be in the form of a cartridge assembly that is latched inside the drilling fluid return riser. In this case the side outlet may be on a separate spool or outlet on the riser.

The demands made on modern RCDs are pushing the envelope and limit of what is achievable with elastomeric seal solutions. The trend is for RCDs to be able to provide effective sealing at higher pressures and higher rotational speeds. Advances in the drill string rotation equipment like top drives, used to rotate the drill string and hence the drilling bit, are allowing revolution rates as high as 300 rpm. There is a desire to be able to use RCDs at much higher pressures during well control operations to enable drill string rotation so as to avoid getting stuck which is a common problem. These types of operations could be carried out if the dynamic rating of the seal solution was comparable to the static housing pressure rating of the RCD which is typically 5000 psi for the high-pressure variants. Furthermore, there is a need to increase the service interval for changing out the seal assembly which is typically failing in less than 200 hours. Premature seal assembly failure leads to drilling fluid invasion of the bearing assembly with consequent costly failure.

The problem is being continuously addressed by novel ways of arranging the seals such as disclosed in U.S. Pat. No. 9,284,811 assigned to Schlumberger Technology Corporation. Another method is to force lubricate the bearings and seals of which there are many examples a recent one being the U.S. Pat. No. 10,066,664 assigned to Black Gold Tools, Inc.

Most modern high-pressure RCDs use elastomeric lip seals or hydrodynamic film seals with the most common ones in use being the wave seals by KALSI™ seals, marketed by Kalsi Engineering, Inc. of Sugar Land, Tex., USA. The only way to improve the performance of seal assemblies using these types of seals is by reducing the pressure and velocity (PV) experienced by such seal assemblies. PV value is a seal design number calculated by the Pressure in psi multiplied by the surface Velocity of the application. Taking a typical rpm of 200, a typical RCD mandrel diameter of 9 inches we get a surface Velocity of about 471 sfpm. Considering the modern PV limit of lip seals is around 250,000 it means that it gets difficult to achieve sealing pressures in excess of 500 psi.

Such improvements are the subject of recent applications US20170114606A1 and US20170167221A1, both by Weatherford Technology Holdings. '606 discloses a stepped pressure concept to lower the pressure differential across the seals, thus reducing the PV per seal. The other application, '221 discloses a split seal assembly that reduces the individual velocity across each seal by reducing the ratio of rpm by use of independently rotating rings which also reduces the total PV seen per seal. Additionally, these design use methods for pressurizing the internal lubrication fluid for the bearings so as to provide reduced pressure differentials. The drawback of using the Kalsi type seal solution is that they must continuously weep to lubricate the interface and that they are harder seals than lip seals leading to wear on the rotating mandrel which eventually also causes leaks.

There is a need for a seal solution which is superior to the complex elastomeric, staged pressure and staged velocity solutions being proposed by the current state of the art. It is an object of the disclosed invention to provide an improved sealing system for RCDs, and furthermore, one which may enable a RCD to operate without a pressurized lubrication circuit or housing.

SUMMARY OF INVENTION

A RCD with a mechanical seal design that enables lubrication of the mechanical seal and prevents drilling mud contamination of the seal. Furthermore, design is disclosed to enable cooling of the mechanical seal. Methods of ensuring the optimal contact mechanism for the mechanical seal faces to accommodate RCD mandrel run out and vibration are disclosed.

According to a first aspect of the invention, we provide a rotating control device for use in a drilling system, the rotating control device comprising a non-rotating tubular RCD housing enclosing an elongate passage, a mandrel which extends along the elongate passage, the mandrel having an axis and being configured in use to rotate about said axis, and a seal assembly which is configured to provide a substantially fluid tight seal between the RCD housing and the mandrel, wherein the seal assembly comprises a first seal ring which is fixed around an exterior face of the mandrel for rotation with the mandrel, and a second seal ring which is secured to the RCD housing, the first and second seal rings each having a sealing face, and a biasing assembly which acts on one of the seal rings to push the sealing faces together by means of a biasing force which is generally parallel to the axis of the mandrel.

In one embodiment, the sealing faces extend generally perpendicular to the axis of the mandrel.

In one embodiment, the biasing assembly is located between the RCD housing and the second seal ring.

In one embodiment, the second seal ring is mounted on a floating seal carrier which has a first seal to provide a substantially fluid tight seal between the RCD housing and the seal carrier.

In one embodiment, the floating seal carrier further comprises a second seal between the mandrel and the seal carrier and a vent passage which extends through the seal carrier from a volume enclosed by the seal carrier, the seal assembly and the mandrel to a low pressure region, the second seal being configured such that the seal carrier does not rotate with rotation of the mandrel.

In one embodiment, the biasing assembly is mounted between the floating seal carrier and the second seal ring.

In one embodiment, the first and second seal rings are both made from a substantially rigid material.

In one embodiment, one or each of the first and second sealing faces comprises a ceramic, tungsten carbide, tungsten disulphide, silicon carbide, diamond or diamond like carbon.

In one embodiment, the rotating control device comprises a separator which, with the seal assembly and RCD housing, contains a lubricant cavity around the mandrel.

In one embodiment, the lubricant cavity is filled with a liquid lubricant or grease.

In one embodiment, the lubricant comprises particles small enough to pass between the first and second sealing faces.

In one embodiment, the separator comprises a further seal carrier having a first seal which provides a substantially fluid tight seal with the RCD housing, and a second seal which provides a substantially fluid tight seal with the mandrel, the further seal carrier having at least one inlet passage to allow for flow of fluid into the lubricant cavity from the side of the seal carrier opposite to the lubricant cavity.

In one embodiment, the second seal of the further seal carrier is configured such that the further seal carrier does not rotate with rotation of the mandrel.

In one embodiment, the rotating control device further comprises at least one baffle which provides an extended flow path for fluid entering the lubricant cavity from the inlet passage to the seal rings.

In one embodiment, the rotating control device further comprises a diaphragm which extends between the RCD housing and the separator so that the RCD housing, seal assembly, diaphragm and separator enclose the lubricant cavity.

In one embodiment, the first seal ring is mounted on a thermally conductive ring which is fixed to the mandrel.

In one embodiment, the interface between the first seal ring and the thermally conductive ring is filled with a thermally conductive material.

In one embodiment, the mandrel is provided with at least one fin which moves fluid into which the fin extends during rotation of the mandrel.

In one embodiment, the fin is configured such that, during rotation of the mandrel in a first direction, the fin causes flow of the fluid along the mandrel towards the seal assembly, radially outwardly from the mandrel towards the RCD housing, and along the RCD housing away from the seal assembly.

In one embodiment, the fin is configured such that during rotation of the mandrel in a second direction, the fin causes flow of the fluid along the RCD housing towards the seal assembly, radially inwardly from the RCD housing to the mandrel, and along mandrel away from the seal assembly.

In one embodiment, the thermally conductive ring is provided with at least one fin which moves fluid into which the fin extends during rotation of the mandrel.

In one embodiment, at least one additional fin which moves fluid into which the fin extends during rotation of the mandrel is provided on the mandrel.

In one embodiment, baffles are mounted on the inside of the RCD housing.

In one embodiment, a resilient damper is installed between the first seal ring and the mandrel.

In one embodiment, a resilient damper is installed between the second seal ring and the RCD housing.

In one embodiment, a resilient damper is installed between the second seal ring and the biasing assembly and the floating seal carrier.

In one embodiment, the RCD housing is provided with an outlet port which extends through the RCD housing from the elongate passage to the exterior thereof.

In one embodiment, the mandrel is a cylindrical tube and has an end on which is mounted an annular elastomeric stripper which is configured to seal against and rotate with a drill pipe located inside the mandrel and extending along the axis of the mandrel.

In one embodiment, the first seal ring is located between the stripper and the second seal ring.

In one embodiment, the RCD housing is provided with an outlet port which extends through the RCD housing from the elongate passage to the exterior thereof, the outlet port being located adjacent the stripper.

In one embodiment, the RCD housing is provided with an outlet port which extends through the RCD housing from the elongate passage to the exterior thereof, the outlet port being located further from the seal assembly than the stripper.

In one embodiment, the RCD housing is provided with an outlet port which extends through the RCD housing from the elongate passage to the exterior thereof, the outlet port being located closer to the seal assembly than the stripper.

In one embodiment, the RCD housing is provided with a primary outlet port and a secondary outlet port which both extend through the RCD housing from the elongate passage to the exterior thereof, the primary outlet port being located further from the seal assembly than the stripper, and the second outlet port being located between the main outlet port and the seal assembly.

In one embodiment, the secondary outlet port is connected to the primary outlet port at the exterior of the RCD housing.

In one embodiment, the rotating control device comprises a bearing assembly mounted between the RCD housing and the mandrel to support rotation of the mandrel about its axis.

In one embodiment, the mandrel is a cylindrical tube and has an end on which is mounted an annular elastomeric stripper which is configured to seal against and rotate with a drill pipe located inside the mandrel and extending along the axis of the mandrel, the seal assembly being mounted between the bearing assembly and the stripper.

We also provide a drilling system comprising a rotating control device in accordance with the first aspect of the invention, with a drill string located within the mandrel and extending along the axis thereof

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a prior art rotating control device;

FIG. 2 is cross-sectional view of a rotating control device (RCD) according to the invention showing the location of the seal assembly;

FIG. 3 is a schematic prior art cross section of a mechanical seal solution for a washpipe seal in a top drive of a drilling rig;

FIG. 4 is a schematic illustration of a cross section of an RCD according to the invention;

FIG. 5 is an illustration of a longitudinal cross section through an alternative embodiment of RCD according to the invention;

FIG. 6A is an illustration of a longitudinal cross section through a further alternative embodiment of RCD according to the invention;

FIG. 6B is an illustration of a longitudinal cross-section through the RCD illustrated in FIG. 6A after substantial use:

FIG. 7 is an illustration of a longitudinal cross section through a further alternative embodiment of RCD according to the invention;

FIG. 8 is an illustration of a longitudinal cross section through a further alternative embodiment of RCD according to the invention;

FIG. 9 is an illustration of a longitudinal cross section through a further alternative embodiment of RCD according to the invention;

FIG. 10 is an illustration of a longitudinal cross section through a further alternative embodiment of RCD according to the invention.

DETAILED DESCRIPTION OF THE INVENTIONS

The problems being solved, and the solutions provided by the embodiments of the principles of the present inventions are best understood by referring to FIGS. 1 to 10 of the drawings, in which like numbers designate like parts.

FIG. 1 is a schematic cross section of a typical prior art rotating control device. It will serve to illustrate the common current methods of achieving sealing. We have a RCD with an RCD housing which in this example comprises an upper housing 12 and a lower housing 10. The upper housing 12 has an adapter 30 threaded at 31 to enable a clamp 22 to connect the upper housing to the lower housing. This is a usual arrangement for land RCDs. It should be appreciated, however, that the assembly may be in one piece and latched into a drilling riser below the slip joint for a floating drilling rig or latched into a diverter just above the BOP on a jack-up drilling rig.

A drillpipe 20 is shown running through the RCD assembly and sealed with a stripper 14 which is mounted on an end of the RCD mandrel 38. In the lower housing 10, there is a side outlet 27 with a seal groove 25 and stud holes 29 for bolting on a side outlet adapter. The pressure load from the stripper 14, due to pressure in the annular space around the exterior of the drillpipe 20 (cavity 15) when drilling with pressure, is transmitted via a load shoulder 17 on the RCD mandrel 38 via a spacer ring 28. The mandrel 38 is supported in the RCD housing by a bearing assembly which in this example comprises two sets of conical roller bearings 16 lower and 18 upper with a spacer sleeve 36. The mandrel 38 is free to rotate with the drillpipe 20 as the drillpipe 20 rotates and frictionally transmits the torque through the stripper 14, to the mandrel.

The upper part of the housing 12 has a retention plate 24 with a seal carrier 26 below it which is sealed with a static seal 23 to the housing 12. A dynamic seal 21 seals the bearing cavity 13 from the outside environment. Seal 21 may be a sealing system consisting of multiple seals like an excluder seal and a dynamic seal. A similar seal carrier 32 seals the bearing cavity 13 against the wellbore pressure in cavity 15. It will have a similar static seal 35 as for the upper carrier and a dynamic seal assembly 33 which can have one or more seals. The exact solution depends on the design of the RCD and whether there is pressurized oil supply to cavity 13 or just a pre-charge pressure in cavity 13. The sealing solutions become very complex as the PV limits of KALSI or lip seals are reached as illustrated in the prior art references.

FIG. 2 shows an RCD according to the first aspect of the invention with a mechanical seal assembly 34 (shown schematically) positioned just below the lower seal carrier 32. It is the intent of this invention that the mechanical seal within the seal assembly 34 takes the full pressure differential from the wellbore cavity 15 to the external environment with no pressure staging in the bearing cavity 13. As such the dynamic seals 21 a and 21 b on upper seal carrier 26 and lower seal carrier 32 will be the same as they are not sealing any pressure differential. The outer static seals 23 a and 23 b will also be the same. This allows for a much simpler design of the RCD as the need for complex PV reduction strategies is removed.

To successfully design a mechanical seal suitable for this application some obstacles need to be overcome and that is the subject of the novel solution presented here. The problems that need to be solved are best illustrated by comparing a prior art application of a mechanical seal to a washpipe with the proposed new invention.

Referring now to FIG. 3, we see a schematic view of a prior art mechanical seal as documented in U.S. Pat. No. 7,343,968 assigned to Deublin and sold as a commercial product by National Oilwell Varco (NOV) as a “Mechanical Washpipe”. It is used to replace soft seal solutions for the rotary seal requirement in the top drive of drilling rigs. The application is rated for 7500 psi and a 4 inch nominal bore. The top drive rotates the drillpipe so the rpm requirement is for 0 to 300 rpm which will be the same rpm for a rotating control device as this is rotated by the frictional torque transfer from the drillpipe 20 to stripper element 14 which turns the RCD mandrel 38 as explained in relation to FIG. 1.

In FIG. 3 we have an upper mechanical seal carrier 101 that connects to the stationary part of the top drive. The lower mechanical seal carrier 102 holds the lower mechanical seal ring 105 and they both rotate as a unit. A floating assembly 103 provides constant pressure onto the upper mechanical seal ring 104 and allows for vibration and run out (shaft oscillation). Drilling fluid is being pumped through the bore 110 of the assembly as depicted by arrow 106. The mechanical seal works by allowing an extremely small amount of fluid, which can be in the form of vapor, to leak from the high pressure cavity 110 to the atmosphere outside 111. This leakage path is shown by arrows 107 and occurs circumferentially across the whole seal interface. The gap in a mechanical seal interface is so small that it will not allow solid particles to pass and it may only allow fluid vapor to pass. Of course, if the seal starts leaking much more rapid flow will occur at arrows 107 and this will spray directly into the atmosphere for this application.

Drilling mud has many small particles of bentonite in it and it is an intrinsic property of drilling fluid to build up a filter cake as the liquid content leaks into the subterranean formation being drilled. This is a desirable property as this filter cake builds up a pressure resistant boundary. The mud particles are too large to fit through the extremely tight gap. Only fluid, so water or oil depending on mud type, will pass through the mechanical seal interface thus lubricating it. Obviously for the functionality of a mechanical seal this type of build-up shown theoretically as a ring deposit of filter cake 108 at the mechanical seal interface is not desirable. However, the high flow velocity of drilling mud through the bore 110, with drilling rates in the order of 400 to 1200 gpm typical on land and 600 to 1800 gpm offshore will easily wash away any such deposits thus assuring the continuous small leakage required for the functioning of the mechanical seal. This leak rate is typically less about 4.2 US fluid ounces per day (125 ml/day) as mentioned in the NOV manual. The high fluid flow rate through mechanical seal also serves to cool it. The system relies on the circulation of mud to cool and lubricate the mechanical seal and the NOV manual warns that extreme wear and failure can occur after 5 minutes of dry rotation.

Referring now to FIG. 4, which shows a schematic cross section view of such a mechanical seal assembly installed in a rotating control device (RCD), in accordance with the invention. We have an RCD housing 151 with a rotating mandrel 152 that is being rotated about its longitudinal axis by a drillpipe 150 via the stripper element 154. Drilling fluid is being pumped through the drillpipe as shown by arrow 155. The drilling fluid travels down to the drill bit (not shown) exits and returns back through the annulus between the drillpipe and the wellbore eventually reaching the annular space in the around the drill pipe 150 in RCD housing 151-RCD cavity 210, and exiting through the side outlet as shown by arrow 206. The side outlet continues as a pressurized line (not shown). The drilling fluid rates and pressure in the pipe 150 are the same conditions as for the washpipe discussed in FIG. 3. However, the drilling fluid return pressure will be lower, and the temperature of the drilling fluid will be higher from heat transfer occurring in hotter subterranean formations. For the purposes of this discussion we will focus on a land application as this will be the most difficult due to the returning heat. The returning drilling fluid will have drill cuttings with it that can range in size from hundredths of an inch up to chunks of an inch in diameter.

The seal assembly comprises a first seal ring 205 which is fixed around an exterior face of the mandrel 152 for rotation with the mandrel 152, and a second seal ring 204 which is secured to the RCD housing 151, the first and second seal rings 205, 204 each having a sealing face. The seal assembly further comprises a biasing assembly 203 which acts on one of the seal rings 205, 204 to push the sealing faces together by means of a biasing force which is generally parallel to the axis of the mandrel 152. In this example, the biasing assembly 203 is located between the outer housing 151 and upper seal ring 204.

In this example, the sealing faces extend generally perpendicular to the axis of the mandrel 152. The first and second seal rings are both made from a substantially rigid material, and one or each of the first and second sealing faces comprises a low friction, wear resistant material or coating—for example a ceramic, tungsten carbide, tungsten disulphide, silicon carbide, diamond or diamond like carbon or other such materials commonly used for mechanical seal rings.

In this example, the RCD includes an upper seal carrier 201. This has a dynamic seal 215 and an outer static seal 216 to stop any fluids entering the bearing assembly cavity 158 which for this application will be not pressurized. The second seal ring 204—hereinafter referred to as the upper mechanical seal ring 204, is attached to the upper seal carrier 201, via the biasing assembly 203, and thus to the outer housing 151 and will be stationary. The first seal ring—hereinafter referred to as lower rotating mechanical seal ring 205 is attached to the rotating mandrel 152 by means of a rigid ring 202 mounted around the exterior of the mandrel 152. This ring 202 may be integral with the mandrel 152.

The seal assembly is required to seal the RCD cavity 210 from the atmosphere 211. In order for the seal assembly to function, it must leak some of the returning drilling fluid to atmosphere. This leak is very small, and it usually comprises the liquid or vapor phase of the drilling fluid, the solid particles being too big to fit through the tight clearance between the seal rings 204,205. To allow passage of this fluid/vapor leak, there is a small annular clearance 213 between the floating seal assembly 203 and the mandrel 152. Any fluid or vapor can exit through small vent passages 214 a and 214 b which extend radially through the seal carrier 201 and RCD housing 151, as shown by arrows 207 a and 207 b. These also serve to act as leak indicator ports in case the mechanical seal starts to deteriorate, in which case substantial flow occurs that will probably include some of the solid particles in the drilling mud.

The further embodiments of the invention illustrated in FIG. 5 onwards include various modifications which may improve the operation of the seal assembly in an RCD as illustrated in FIG. 4.

In the embodiment of FIG. 4, there is no direct cooling of the mechanical seal: the flow in the drillpipe is insulated from the mechanical seal by the annular space 156 between the drill pipe 150 and the mandrel 152, which is a dead space with no circulation except for convection and some minor agitation as the drillpipe is being lowered through the stripper element when drilling and deepening the wellbore, and the main return flow 206 exits below the mechanical seal for this conventional design giving rise to a further dead space 217 in the RCD cavity 210 around the mechanical seal.

The leakage that must occur from the RCD cavity 210 to the exterior of the RCD across the mechanical seal is shown with arrows 209. This will result in some build-up of filter cake circumferentially around the seal shown as debris 208. Some of this debris will be thrown off by the centrifugal motion of the lower mechanical seal ring, but these are not the right conditions for a successful mechanical seal. While the rotational pressure requirement of this seal is less than that of the washpipe, typically 2500 psi compared to 7500 psi, the diameter is about 2.5 times greater than that of a washpipe, making the PV requirements similar and thus achievable with the ceramic type mechanical seal materials being used successfully for washpipes.

Another issue is the existence of a dead cavity 217 which besides having very little circulation can also accumulate drilling gases leading to this cavity being filled with gas over time which will further affect the ability to cool the mechanical seal and could lead to the undesirable dry running condition mentioned earlier as a major failure point of the washpipe mechanical seal. Solutions for the problems of lubrication and cooling will now be presented to ensure a working mechanical seal solution for a rotating control device.

Referring now to FIG. 5, we show a partial cross section of a RCD with a schematic spring energized mechanical seal 50 installed. In this example, the mechanical seal uses a helical compression spring for applying pressure to the mechanical seal interface, but it could equally use bellows or other mechanical seal actuation mechanisms instead of the spring.

The RCD housing 12 has a seal carrier 32 installed that isolates the bearing cavity 45 from the mechanical seal assembly 50, with a static seal 23 against the outer housing and a dynamic seal 21 against the rotating mandrel 38. The lower rotating mechanical seal ring 60 is attached to and rotating with the mandrel 38 against the upper rotating mechanical seal ring 58. The required leak can be exhausted through annular channel 53 and out through ports 51 a and 51 b to the external environment. These ports can also be connected with small bore metallic tubing to a lower pressure downstream of the outlet 27. It is common for land installations to have a pressure control device (not shown) like a control valve or choke bolted to outlet 27 with bolt holes 29 and sealed with a seal ring 25 (FIG. 2). In this case the small bore metallic tubing from ports 51 a and 51 b can be ported back into the flow downstream of such a pressure control device to have a closed system vent for the mechanical seal which is a good solution for preventing emissions to atmosphere from the mechanical seal weep.

There is also a lower seal carrier 62 whose purpose it is to separate the section containing the mechanical seal assembly 50 from the wellbore cavity 40. The seal carrier has a static seal 59 situated in the main housing 12 and a dynamic seal 43 with respect to the rotating mandrel 38. This allows the complete cavity containing the mechanical seal consisting of cavities 52, 48, 46, and 44 to be filled with a grease or purpose specific mechanical seal lubrication fluid. The lubricant may have additive nanoparticles capable of penetrating a mechanical seal barrier having friction reducing properties like nanoparticle Tungsten Di-sulphide or nanoparticle Molybdenum Di-sulphide. The lower seal carrier 62 excludes debris like drill cuttings from the sensitive pre-load mechanism of the mechanical seal but will allow drilling fluid to enter through multiple ports 42 in the lower seal carrier 62, in order to drive the lubrication of the mechanical seal.

There are two stationary annular baffles 56 and 54 that provide a forced displacement path for the mechanical seal lubricant. These lubricants are viscous and lighter than typical drilling fluids so they will stay in place with this baffle arrangement. As lubricant is used up in the vicinity of the mechanical seals rings 58 and 60 (cavity 46), the drilling mud is pushed through the ports 42 into cavity 52, displacing lubricant along cavity 44 towards cavity 46. Cavity 48 which is the cavity containing the moving parts of the mechanical seal 50 is kept full at all times with lubricant. The volume in this labyrinth of annular baffles is sufficient for the maximum intended operating time of the mechanical seal, so there is enough volume with this arrangement for drilling mud to never reach the mechanical seal moving parts or seal ring interface. In this manner a superior operating environment is created for the mechanical seal.

A lubricant replenishment port 57 is provided in the RCD housing just below the upper seal carrier 32, for re-filling the baffle arrangement and pushing any drilling mud back out again. This would be a prudent procedure to carry out any time the bearing assembly is removed from the housing, for example when changing out the drilling bit due to wear.

In FIGS. 6a and 6 b, an alternative arrangement for ensuring lubrication to the mechanicals seal is depicted using a single annular baffle 74 and a flexible diaphragm 76. Like parts with FIG. 5 are numbered the same. In FIG. 6a we show a slightly different lower seal carrier 70 which is sealed to the outer housing 12 with a static seal 75 and a dynamic seal 72 against the rotating mandrel 38. This has multiple ports 79 which allow drilling mud from cavity 40 to enter the diaphragm cavity 77. When the unit is first assembled and prepared for deployment the internal cavities 46,48 and 52 are fully loaded with grease or specific mechanical seal lubricant. Annular cavity 47 roughly defines the available lubricant volume that can be displace by the diaphragm with mud behind it.

FIG. 6b shows the diaphragm fully displaced which is the maximum lubrication fluid volume available for keeping the mechanical seal faces lubricated. Cavity 47 has now been taken up by mud, which drives lubricant upwards on the outer diameter of the annular baffle 74 along annular cavity 52. The lubricant then crosses the top of baffle 74 and then travels downward along cavity 48 and supplies lubricant to cavity 46 just adjacent to the seal rings 58 and 60 thus ensuring continuous lubrication. The lubricant can be replenished via grease port 57 which allows pumping the diaphragm back to push out the mud and preparing the system for further continued use.

Referring to FIG. 7 an embodiment is presented to cool the mechanical seal interface. The like parts with FIG. 5 are numbered the same. The support ring 80 for the lower seal 62, is made out of thermally conductive material. A highly thermally conductive grease or similar material is placed at the interface 81 between the lower seal ring and support ring heat sink 80.

Fins 87, preferably integral with the thermally conductive ring 80, make this act as an impeller that can move drilling fluid over the surface of the heat conductive ring, the fins also acting as radiators for conducting heat away from the lower mechanical seal. The fins 87 can be orientated such that when the rotating mandrel 38 turns clockwise, the normal direction for drilling that the drilling fluid is radially directed across the seal carrier 62 and then down along the inside circumference of the outer housing 12. This flow may also be induced in the reverse direction i.e. up the inner circumference of the housing radially inwards across seal carrier 62 and down the mandrel 38, by changing the orientation of the fins 87, if this is deemed to be the more efficient flowpath for cooling. Additional fins 88 may be clamped onto the mandrel 38 to facilitate the cooling circulation. One or more annular baffles 90 may be mounted with rods 86 to further force a predetermined flow path for optimal heat transfer away from the mechanical seal.

Referring to FIG. 8 an embodiment is disclosed with additional features to assist in the optimal performance of a mechanical seal. During drilling significant temperature changes can occur in the RCD due to the start and stop nature of the process as well as the changes in drilling fluid temperature. Vibration due to induced drillstring harmonics while drilling can occur. Load differences on the stripper rubber due to pressure and direction of movement of the drillpipe either up or down can occur. All of these variables can contribute to variations in run out and/or vibration of the RCD mandrel. While some may be accommodated by the usual design of mechanical seals using variations of floating mounts there may be a need to further enhance these designs for the RCD application. In FIG. 8 we show a resilient damper 82 installed between the lower seal carrier 32 and the upper part 85 of the mechanical seal assembly 50. This resilient damper 82 is made from flexible and resilient material, which can accommodate additional horizontal or vertical movements as required or act as a damper for vibration. A similar resilient damper 84 can be used between the lower seal ring 62 and RCD mandrel 38 to reduce adverse operating conditions on the mechanical seal interface.

The flexible mounting could be made from an elastomeric material such as rubber, polyurethane or an optimally designed matrix with fiber reinforcement to give the required flexibility and resilience.

Referring to FIG. 9 which shows an embodiment of the invention using a mechanical seal solution of the type described in FIG. 7 with a second outlet 96 just below the mechanical seal assembly 50. This will allow some drilling fluid to exit at this point to aid with cooling. It is a known problem during longer drilling intervals or after being slightly underbalanced with respect to the formation being drilled, that gas coming out of solution can accumulate in the dead space between the rotating stripper element and the bottom of the seal assembly. This can adversely affect the heat transfer in this region. Adding an additional line 92 will resolve this problem. The line will work because the main flow exiting from outlet 27 will cause a pressure drop at outlet 98 of secondary line 92 which will induce flow from this secondary outlet. This can be further enhanced by using the impeller fins 88 in combination with an annular baffle ring 90 that is closed top and bottom with plates 91 and 93 to create a pumping mechanism that draws drilling fluid from below the stripper rubber 14 from the cavity 15 and forces it out at outlet 96 into the secondary outlet 92. In this way a continuous supply of drilling fluid is passed along the fins 87 attached to the thermally conductive ring 80 just below the lower mechanical seal ring 60. This will ensure an effective cooling circuit for the mechanical seal interface.

In FIG. 10 an alternative solution to the problem of gas build up and cooling is presented. A mechanical seal solution with a diaphragm as described in FIG. 6A is shown. The body 11 of the lower housing is widened to allow unfettered flow past the rotating stripper element 14 with the fluid exit 27 being above the rotating stripper element 14 which is a solution that has not been used to date in any prior art. This will allow significant volume of drilling fluid flow past the bottom of the mechanical seal thus ensuring optimal cooling.

Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

It is therefore contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention. 

What is claimed is:
 1. A rotating control device for use in a drilling system, the rotating control device comprising: a non-rotating tubular RCD housing enclosing an elongate passage; a mandrel which extends along the elongate passage, the mandrel having an axis and being configured in use to rotate about said axis; and a seal assembly which is configured to provide a substantially fluid tight seal between the RCD housing and the mandrel, wherein the seal assembly comprises: a first seal ring which is fixed around an exterior face of the mandrel for rotation with the mandrel, and a second seal ring which is secured to the RCD housing, the first and second seal rings each having a sealing face, and a biasing assembly which acts on one of the seal rings to push the sealing faces together by means of a biasing force which is generally parallel to the axis of the mandrel.
 2. A rotating control device according to claim 1, wherein the sealing faces extend generally perpendicular to the axis of the mandrel.
 3. A rotating control device according to claim 1, wherein the biasing assembly is located between the RCD housing and the second seal ring.
 4. A rotating control device according to claim 1, wherein the second seal ring is mounted on a floating seal carrier which has a first seal to provide a substantially fluid tight seal between the RCD housing and the seal carrier.
 5. A rotating control device according to claim 1, wherein the floating seal carrier further comprises a second seal between the mandrel and the seal carrier, and a vent passage which extends through the seal carrier from a volume enclosed by the seal carrier, the seal assembly and the mandrel to a low pressure region, the second seal being configured such that the seal carrier does not rotate with rotation of the mandrel.
 6. A rotating control device according to claim 4, wherein the biasing assembly is mounted between the floating seal carrier and the second seal ring.
 7. A rotating control device according to claim 1, further comprising a separator which, with the seal assembly and RCD housing, contains a lubricant cavity around the mandrel.
 8. A rotating control device according to claim 7, wherein the separator comprises a further seal carrier having a first seal which provides a substantially fluid tight seal with the RCD housing, and a second seal which provides a substantially fluid tight seal with the mandrel, the further seal carrier having at least one inlet passage to allow for flow of fluid into the lubricant cavity from the side of the seal carrier opposite to the lubricant cavity.
 9. A rotating control device according to claim 8, wherein the second seal of the further seal carrier is configured such that the further seal carrier does not rotate with rotation of the mandrel.
 10. A rotating control device according to claim 8, further comprising at least one baffle which provides an extended flow path for fluid entering the lubricant cavity from the inlet passage to the seal rings.
 11. A rotating control device according to claim 7, further comprising a diaphragm which extends between the RCD housing and the separator so that the RCD housing, seal assembly, diaphragm and separator enclose the lubricant cavity.
 12. A rotating control device according to claim 1, wherein the first seal ring is mounted on a thermally conductive ring which is fixed to the mandrel.
 13. A rotating control device according to claim 1, wherein the mandrel is provided with at least one fin which moves fluid into which the fin extends during rotation of the mandrel.
 14. A rotating control device according to claim 12, wherein the thermally conductive ring is provided with at least one fin which moves fluid into which the fin extends during rotation of the mandrel.
 15. A rotating control device according claim 1, wherein baffles are mounted on the inside of the RCD housing.
 16. A rotating control device according to claim 1, wherein a resilient damper is installed between the first seal ring and the mandrel.
 17. A rotating control device according to claim 1, wherein a resilient damper is installed between the second seal ring and the RCD housing.
 18. A rotating control device according to claim 6, wherein a resilient damper is installed between the second seal ring and the biasing assembly and the floating seal carrier.
 19. A drilling system comprising a rotating control device in accordance with claim 1 with a drill string located within the mandrel and extending along the axis thereof. 